LUCINA. 47 



ventral margin is strongly arcuated, and usually, but not 

 invariably, rises rather the more in front, thus causing the 

 anterior side to be rather the less broad. The front dorsal 

 edge is more or less retuse, and declines in but a trifling 

 degree ; the hinder dorsal edge is nearly straight, or hardly 

 convex, and scarcely slopes at all. The anterior side, 

 which exhibits a rounded off angle above, but not the 

 slightest angulation below, is decidedly, but not so very 

 greatly, the shorter ; the posterior side is angulated above, 

 and, similarly to the other side, is destitute, except in 

 young individuals, of any inferior angulation. The urn- 

 bones project but little ; the beaks are acute, and lean a 

 little forward ; in front of them is a shallow lanceolate 

 lunule. The ligament is broad and large, but not promi- 

 nent, and is usually of a rather pale colour. The internal 

 surface is of a more or less dull white ; the hinder muscular 

 scar is very large, and the margin perfectly free from crena- 

 tion. There exists in each valve a bifid and a simple primary 

 tooth, both of which are of moderate size ; and in the right 

 valve a very small not very distant anterior lateral one, 

 which, however, is often very obscure. 



The breadth of a fair-sized specimen was an inch and a 

 half, which measurement was very slightly exceeded by its 

 length. 



We are indebted to Mr. Alder for a sketch of the 

 animal of this species, one of the many obligations con- 

 ferred on us by our much valued friend. It is accompanied 

 by the following note : " I enclose you a drawing of Lucina 

 borealis, made from specimens sent me by Mr. Embleton. 

 During the ten days I kept them alive, they shewed no 

 more than I have represented, from which I conclude that 

 the two siphons are never exserted. The mantle is widely 

 open in front, with the margins waved or plicated ; there 



